fix optimization option wrongly scoped

lib/less/parser.js

@@ -52,6 +52,8 @@ less.Parser = function Parser(env) {
         inputLength,
         parser;
 
+    var that = this;
+
     // This function is called after all files
     // have been imported through `@import`.
     var finish = function () {};
@@ -187,8 +189,8 @@ less.Parser = function Parser(env) {
             // Either delimited by /\n\n/ or
             // delmited by '\n}' (see rationale above),
             // depending on the level of optimization.
-            if (this.optimization > 0) {
-                if (this.optimization > 2) {
+            if (that.optimization > 0) {
+                if (that.optimization > 2) {
                     input = input.replace(/\/\*(?:[^*]|\*+[^\/*])*\*+\//g, '');
                     chunks = input.split(/^(?=\n)/mg);
                 } else {